1. Field of the Invention
The present invention relates to a computer program product, system, and method for point-in-time snap copy on asynchronous consistency group management.
2. Description of the Related Art
Data storage systems, particularly at the enterprise level, are usually designed to provide a high level of redundancy to reduce the risk of data loss in the event of failure of a component of the data storage system. Thus, multiple copies of data are frequently stored on multiple systems which may be geographically dispersed. Thus, data from a host to be stored in the data storage system is typically directed to a primary system at a local site and then replicated to secondary system which may be geographically remote system from the primary system.
The process of replicating, that is, copying data over to the secondary system can be setup in either a synchronous or asynchronous relationship between the primary system and the secondary system. In a synchronous relationship, any updates to the primary system are typically synchronized with the secondary system, that is, successfully copied over to the secondary system, before the primary system reports to the host that the data storage input/output operation has been successfully completed. As a result, data storage to a primary storage system and a secondary system in a synchronous relationship may adversely affect system performance of the host while the host awaits completion of the synchronous copy operations. In storage systems in which the primary and secondary systems are widely separated, delays to completion of the synchronous copy operations may be greater, which can further adversely affect host performance. For example, in storage systems in which the primary system and the secondary system are separated by a geographical distance of 300 kilometers (Km) or more, for example, there can be a delay of 3-5 seconds (or more) before the data is successfully replicated at the secondary system.
As a result, storage systems frequently employ an asynchronous relationship between a primary system and a secondary system, particularly if the secondary system is geographically distant from the primary system. In an asynchronous relationship, successful updates to the primary system are typically reported to the host as a successful storage input/output operation without waiting for the update to be replicated to the secondary system. As a result, the host need not wait for the data replication to the secondary system to be completed, and the performance of the host system may be enhanced accordingly.
As the name suggests, in an asynchronous relationship, the primary system and the secondary system may not be fully synchronized at any one time. Thus, data stored on the secondary system typically lags that stored in the primary system. Accordingly, new data stored on a primary system may not be stored on the secondary system for an interval of time such as 3-5 seconds, for example.
In data replication systems, data is typically maintained in volume pairs, comprising a primary volume in a primary storage device and a corresponding secondary volume in a secondary storage device that includes an identical copy of the data maintained in the primary volume. The primary and secondary volumes are identified by a copy relationship in which the data of the primary volume, also referred to as the source volume, is copied to the secondary volume, also referred to as the target volume. Primary and secondary storage controllers may be used to control access to the primary and secondary storage devices.
A near instantaneous copy of a volume may be generated using a point-in-time snap copy function such as the IBM® FlashCopy function, for example. The point-in-time snap copy function creates a “snapshot” of the contents of a source volume as of a particular point-in-time in a target volume which may be referred to as the point-in-time snap copy volume. One version of a point-in-time snap copy function transfers the contents of the source volume to the point-in-time snap copy volume in a background copy operation.
A local copy target bitmap having a bit for each track of the point-in-time snap copy volume, is used to indicate tracks which have not yet been transferred to the point-in-time snap copy volume. As the contents of each track of the source volume is copied to the target volume, the corresponding bit of the local copy target bitmap is updated (typically reset) to indicate successful transfer of the contents for the associated track. Any read operations directed to a track of the point-in-time snap copy volume which has not yet received the contents of the corresponding track of the source volume, are redirected to obtain the contents of that track from the source volume. Accordingly, the contents of a point-in-time snap copy volume are immediately available albeit indirectly, before any tracks have actually been transferred to the target volume. Conversely, if the host directs an update to a track of the source volume before the contents of that track have been transferred to the point-in-time snap copy volume, the contents of the track of the source volume are transferred to the point-in-time snap copy volume before the update is permitted to overwrite the contents of that track of the source volume.
Another version of a point-in-time snap copy function omits the background copy operation. Thus, the contents of the source volume are not transferred to the point-in-time snap copy volume in a background copy operation but are transferred in response to an update of the source volume. Accordingly, any read operations directed to a track of the point-in-time snap copy volume are usually redirected to obtain the contents of that track from the source volume. However, if the host directs an update to a track of the source volume, the contents of the track of the source volume are transferred to the point-in-time snap copy volume before the update is permitted to overwrite the contents of that track of the source volume.
In a synchronous data replication system, a point-in-time snap copy volume may be made of a primary volume at the primary system with respect to the contents of the primary volume as of a particular point-in-time, and a point-in-time snap copy volume may be made of a secondary volume at the secondary system with respect to the contents of the secondary volume as of the same point-in-time. Because the primary and secondary volumes are synchronized, the respective point-in-time snap copy volumes will also be synchronized and therefore consistent with each other notwithstanding that the point-in-time snap copy volumes are made in different places, that is, the primary system and the secondary system.
In an asynchronous data replication system, a point-in-time snap copy volume may also be made of a primary volume at the primary system with respect to the contents of the primary volume as of a particular point-in-time. Similarly, a point-in-time snap copy volume may be made of a secondary volume at the secondary system with respect to the contents of the secondary volume as of a particular point-in-time. However, because the primary and secondary volumes are not synchronized, the respective point-in-time snap copy volumes will also not be synchronized and therefore will not be consistent with each other. Hence, to provide a backup copy of a point-in-time snap copy volume of a primary volume at the primary system, the point-in-time snap copy volume generated at the primary system is typically transferred over a network to the secondary system.
To maintain a degree of consistency of data across multiple volumes at a secondary system, the IBM® TotalStorage disk subsystem Peer-to-Peer Remote Copy (PPRC) function supports the concept of a PPRC consistency group. Volumes in a PPRC relationship that are configured into a PPRC Consistency Group are maintained to ensure that a group of updates made to volumes at the primary system are also eventually made on the volumes at the secondary system to maintain data consistency for those volumes of the group. Accordingly, consistency groups may be formed between volumes of the primary system and the secondary system which are consistent with respect to a particular set of updates or a particular point in time, notwithstanding the overall asynchronous relationship between the primary system and the secondary system.
One of the volumes of the primary system which may be in a consistency group with volumes at the secondary system, may itself be a target of a point-in-time snap copy as long as the point-in-time snap copy relationship is initiated prior to formation of the consistency group which includes the target volume of the point-in-time snap copy. Previously establishing a point-in-time snap copy relationship having a target volume which is already one of the volumes of the primary system in a consistency group with volumes at the secondary system, is not supported.